Judicious therapeutic use of antimicrobials in aquatic animal medicine

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Introduction

Aquatic animal veterinarians should use all therapeutants, including antimicrobials, judiciously to restore aquatic animal health, ensure the continued production of cultured safe seafood, and minimize development of antimicrobial resistance.

In addition to awareness of and adherence to AVMA's Core Principles of Antimicrobial Stewardship in Veterinary Medicine and Judicious Therapeutic Use of Antimicrobials guidelines, veterinarians should follow these specific principles and recommendations for antimicrobial use in aquatic animals:

I.    Disease prevention in aquatic animals

  • Help clients design health management programs that will reduce the incidence of disease, and the need for antimicrobial therapy.
  • Work with clients and facility personnel to develop written training tools and standard operating procedures on quarantine, diagnostic testing, water quality monitoring, judicious antimicrobial use, withdrawal times, routes of administration, storage, handling, and record keeping.
  • Develop biosecurity plans based on risks of pathogen introduction and transmission, especially if the captive population shares water sources with wild populations.

II.    Disease diagnosis in aquatic animals

  • Make a thorough assessment to identify root cause(s) and develop appropriate mitigation measures to address predisposing factors when there is an increased disease incidence. This could involve an evaluation of husbandry and routine health management practices such as nutrition, stocking density, vaccination, health assessment and water quality.
  • Submit diagnostic samples to a diagnostic laboratory familiar with the species of aquatic animal being evaluated. Assess clinically ill or recently dead aquatic animals with tests such as gross pathology and histopathology, culture for microbial agents, and molecular assays. This will assist in developing therapy and control protocols.

III.    Antimicrobial selection, use, and therapy in aquatic animals

  • Issue prescriptions and Veterinary Feed Directives (VFD) only in the context of a veterinarian-client-patient relationship (VCPR).
  • Select and use antimicrobials based on feeding rates and clinical evidence of the etiology of the disease, history, clinical signs, gross lesions, and/or diagnostic laboratory data. Clinical findings and diagnostic test records on individual animals or populations can help formulate effective therapy using antimicrobials. Fish fed infrequently or at low biomass rates are not candidates for medication with antimicrobial feed.
  • Determine in vitro antimicrobial susceptibility of bacterial pathogens isolated from cultures. For most aquatic animal bacterial pathogens, minimal inhibitory concentration (MIC) and disc diffusion breakpoints have not been standardized. Therefore, veterinarians should contact regional diagnostic laboratories with aquatic disease specialists for assistance with interpretation of provisional susceptibility results. Veterinarians should work with fish farm managers to monitor the clinical therapeutic response and to evaluate the in vivo efficacy of antimicrobials.
  • Use antimicrobials in accordance with the product label (i.e., indication, dosage, frequency, duration, route of administration, species, and environmental conditions and requirements) although exceptions (as described below) can occur. Protect antimicrobials' integrity through proper handling, storage, observation of the expiration date, and recordkeeping.

Considerations for antimicrobial use in aquatic species include:

  1. Before authorizing antimicrobial administration, the veterinarian should remind the production facility to comply with applicable federal, state, and local laws and regulations (eg. National Pollutant Discharge Elimination System permits).
  2. Use of Veterinary Feed Directive antimicrobials in aquatics should follow the Veterinary Feed Directive rules.
    1. Preslaughter withdrawal times in aquatic animals intended as food should be observed.
    2. VFD antimicrobial feeds must be used prior to the expiration of the VFD order. If the order expires prior to completion of the duration of use, the veterinarian must issue a new VFD.
  3. FDA allows regulatory discretion for extralabel use of VFD medicated feed for minor species, including all aquatic species, provided specific conditions of CPG 615.115 are met.

Conclusion

Many factors influence the incidence and outcome of disease outbreaks, disease diagnosis, and disease therapy in aquatic species. Antimicrobials can play an important role in aquatic animal medicine but they must be used in a judicious and legal manner. Resources such as the AVMA or the American Association of Fish Veterinarians can help navigate the judicious and legal use of antimicrobials as well as the unique aspects of aquatic animal production, management, and therapy.

References

  1. Clinical and Laboratory Standards Institute (CLSI), Methods for Antimicrobial Broth Dilution and Disk Susceptibility Testing of Bacteria Isolated From Aquatic Animals, 2nd edition. CLSI guideline VET03. Wayne, PA: Clinical and Laboratory Standards Institute; 2020.
  2. Clinical and Laboratory Standards Institute (CSLI). Performance Standards for Antimicrobial Susceptibility Testing of Bacteria Isolated From Aquatic Animals, 3rd edition. CLSI supplement VET04. Clinical and Laboratory Standards Institute 2020.
  3. David R. Smith; et al. (2019) The AVMA’s definitions of antimicrobial uses for prevention, control, and treatment of disease. Journal of the American Veterinary Medical Association, Vol. 254 (7): 792-797.
  4. Compliance Policy Guide Sec.615.115 Extralabel Use of Medicated Feeds for Minor Species
  5. Veterinary Feed Directive 80 FR 31717-31725
  6. What veterinarians need to know about antimicrobial susceptibility testing.

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